The present invention relates to an electrophotographic type image forming apparatus such as a copier, printer, facsimile device, or similar device, in which a charging means, an image writing means, and developing means are arranged around an image carrier, and a toner image formed on the image carrier is transferred onto both surfaces of transfer material and fixed.
Conventionally, in two-sided image formation, an image forming method is used in which a one-side image formed on the image carrier is transferred onto a transfer material and fixed; the transfer sheet is temporarily accommodated in a two-sided reversal sheet feeding device; the transfer material is fed from the two-sided reversal sheet feeding device in timed relationship with the image formed again on the image carrier; and the other side image is transferred on the transfer material and fixed.
As described above, in this two-sided image forming apparatus, conveyance of the transfer material such as feeding to the two-sided reversal sheet feeding device, and two times passage through the fixing device, is carried out. Accordingly, reliability of the transfer material conveyance is low, and jamming of the transfer material or shrinkle is sometimes caused. In addition, as is commonly known, conveyance length of the transfer material becomes larger. Therefore, there was a problem that it takes much time for copying.
With respect to this, a method in which fixing is carried out only once after toner images have been formed on two sides of the transfer material, is proposed in Japanese Patent Publication Nos. 37538/1974, 28740/1979, and Japanese Patent Publication Open to Public Inspection Nos. 44457/1989, and 214576/1992.
The inventors of the present invention are investigating an image forming apparatus and an image forming method, in which a plurality of sets of toner image forming means composed of a charging means, image writing means, developing means, etc., are arranged around a photoreceptor drum (the first image carrier means); after superimposed color toner images formed on the photoreceptor drum are temporarily and collectively transferred onto a belt-like toner image receiving body (the second image carrier means) by a first transfer means, superimposed color toner images are formed again on the photoreceptor drum; a transfer material fed in timed relationship with the toner image on the photoreceptor drum and the toner image on the toner image receiving body, is electrically charged by a transfer material charging means, and is attracted to the toner image receiving body; the toner image on the photoreceptor drum is transferred by the first transfer means as an obverse side image, and the toner image on the toner image receiving body is transferred by the second transfer means as a reverse side image, respectively onto two sides of the transfer material conveyed on the toner image receiving body; after that, the transfer material is separated from the toner image receiving body by discharging of a transfer material separation means; the toner images on the transfer material are fixed by a fixing device (fixing means); and a two-sided color image is formed.
As described above, in the above-mentioned image forming apparatus, the number of passage of the transfer sheet on which a toner image is transferred on both sides through the fixing device is allowed to be once. Accordingly, reliability of the transfer material conveyance is high, and conveyance path of the transfer material is shortened, increasing processing speed of image forming.
However, in the above-described image forming apparatus, because the transfer material having unfixed toner images on two sides is conveyed to the fixing device, the toner image receiving body and the fixing device are closely arranged to each other, and the transfer material separated from the toner image receiving body is directly sent to the fixing device. However, when the toner image receiving body is arranged closely to the fixing device, problems occur in which the toner image receiving body is deformed, the toner image transferred onto the toner image receiving body is slightly fused and is hardly transferred, or toner fixedly adheres onto the toner image receiving body. For this reason, the toner image receiving body and the fixing device are arranged with some distance, a conveying section having a spur member (spur) is provided between them, and the transfer material is conveyed from the toner image receiving body to the fixing device by the spur. However, problems occur in which, during transfer material conveyance by the spur, the transfer material is pressed to the direction of the spur, and spur trace occurs on the toner image on the transfer material or the spur is stained by toner, thereby, a toner stain occurs on the transfer material conveyed due to the spur.
The first object of the present invention is to solve the above-described problems, and to provide an image forming apparatus in which generation of a spur trace on a toner image on the transfer material is prevented and toner stain on the transfer material due to stain by spur is prevented.
Further, there is also a problem in which, after the toner images have been transferred onto both sides of the transfer material conveyed by a toner image receiving body, in the case where the transfer material is separated from a toner image receiving body, and the toner images on the transfer material are fixed while the transfer material is being nipped and conveyed by the fixing device, the conveyance speed of the transfer material by the fixing device and that of the transfer material by the toner image receiving body are set to the same speed. However, even if aforesaid conveyance speeds are set to be the same, the transfer material is pulled by a slight speed difference caused between the two conveyance speeds, and unfixed toner images transferred onto two sides of the transfer material are partially disturbed.
The present invention has solved the above problems, and as the result, the second object of the present invention is to provide an image forming apparatus in which, even if a change of speed is generated in the conveyance speed of the transfer material by the second image carrier means or in the conveyance speed of the transfer material by the fixing means, it can be prevented that the transfer material is strongly pulled, and thereby the toner image on the obverse and the reverse side thereof can be transferred and fixed without deteriorating the image quality.